The invention relates to a process for the manufacture of paper, paperboard or cardboard by applying spray starch to a moving, wet sheet of material.
When producing paper, paperboard or cardboard, in the following collectively referred to as paper products, in most cases starch is added to increase strength. Especially when using a more or less large portion of waste paper in addition to fresh pulp, the addition of starch is necessary because reprocessed recycling paper usually has a much shorter average fiber length, which would substantially affect the strength of the paper product. Generally, between 0.5 and 10 wt %, occasionally up to 15 wt %, usually between approximately 1 and 5 wt %, based on the dry weight, of starch is added to pulp suspensions.
The starch may be added as mass starch, spray starch or surface starch. Mass starches mixed with the pulp suspension already in the approach flow system of the paper machine, i.e. in the chest before the headbox; cationic and/or anionic modified starches are frequently used, which are added in the form of a powder, a suspension or, most commonly, a solution.
Spray starch is usually applied as an aqueous suspension, e.g. approx. 3%, after the headbox in the wire section. During the production of multi-layer paper products, the spray starch is often (also) sprayed between individual layers in order to increase layer strength during pasting. The spray starch thus applied penetrates into the material sheet when passing the wire section and again increases the internal strength of the paper product.
Surface starch is usually applied, i.e. rolled, onto the surface of the material sheet in the size press, i.e. in the dry section, in the form of a thin film of an aqueous starch solution. This suspension not only serves to increase the strength, but also to adjust the surface properties, especially smoothness, porosity and printability, and it may also contain a number of additives such as sizing agents, pigments, dyes and optical brighteners.
Of course, the necessity of adding starch considerably reduces part of the economic benefit of using recycling paper or fibers. Due to the fact that starch is a natural polymer with a predetermined grain size distribution, a large portion of the hydrated spray starch grains of the suspension pass the material sheet in the wire section and is withdrawn together with the water. Without an at least rough mechanical separation before the downstream sewage treatment plant, preferably by recovering the starch from the waste water, this spray starch portion represents an additional burden to the sewage treatment plant.
Before this background, the object of the invention was to provide an improved process for the manufacture of a paper product, by which process the above-mentioned disadvantages may be at least partly eliminated.
In the course of his research, the inventor of the present subject matter found out that spray starch may also be applied to the wet sheet of material in solid, i.e. powder, form. The application of solid spray starch is mentioned in DE 2,108,658 A merely in one single sub-claim (but not in the description) and is only implicitly suggested in the WO 2006/037750 A1 and its priority application, DE 10 2004 048 430 A1. Detailed procedures or advantages of using solid spray starch are not disclosed in any of these applications.